This invention relates broadly to liquid-liquid contacting systems and more particularly to systems wherein perforated liquid-liquid contacting trays are employed.
In the practice of contacting immiscible liquids, as for example, in fractionation and extraction processes, it has been common practice in the art to employ vertically oriented contacting columns wherein a multiplicity of horizontal trays are mounted in vertically displaced relationship with respect to each other. These trays thus define a series of inter-tray spaces which constitute the contacting zones in the column wherein the respective liquid phases are brought together in intimate admixture for contacting. In operation, the heavier or denser phase is introduced at the top of the contacting column and the lighter or less dense phase at the bottom thereof, so that the difference in density constitutes a driving force causing the respective liquid phases to flow through the column. Contacted heavier phase liquid is removed from the system at the bottom of the tray column and contacted lighter phase liquid is removed at the top of the column.
The above type of contacting systems frequently employ trays of a kind known as "sieve trays" featuring a multiplicity of discrete perforations in the tray plate, or deck, through which liquid may flow. In operation, one liquid phase (the "discontinuous" phase) is dispersed as droplets as it flows through the perforated deck. The droplets thus formed rain through the other ("continuous") phase in the contacting zone between adjacent trays, collect and coalesce on the adjacent downstream tray, with coalesced discontinuous phase liquid being discharged at the other side of the downstream tray from the perforations therein. In this manner, the discontinuous phase liquid is serially flowed in the form of small droplets through all of the inter-tray contacting zones in the column. Simultaneously, the continuous phase liquid is flowed through the column in a generally countercurrent fashion and transferred between adjacent inter-tray contacting zones by means of channels or conduits associated with each tray. These continuous phase transfer means are termed "downcomers" when the heavier phase is continuous and flows vertically downwardly therethrough and "upcomers" when the lighter phase is continuous and flows vertically upwardly therethrough.
For the purpose of convenience in the ensuing discussion, the discontinuous phase liquid will be taken as referring to the heavier, or denser, phase and the continuous phase will be taken as referring to the lighter, or less dense, liquid phase. Under this terminology, the liquid transfer means associated with the contacting trays will be identified as "upcomers", i.e., these means serve to transfer the lighter continuous phase liquid from a lower inter-tray contacting zone below the given tray to a higher inter-tray contacting zone above the tray. It will be recognized that the foregoing terminology is intended for ease of description only and that the following discussion is in principle equally applicable to heavier continuous phase - lighter discontinuous phase sieve tray column contacting systems having downcomer means for transfer of the continuous phase liquid between adjacent inter-tray contacting zones.
In the sieve tray liquid-liquid contacting system, it is important that the height of the discontinuous phase liquid layer collecting on the perforated deck of the tray be sufficient to allow the droplets of discontinuous phase liquid discharged from the tray above and passed through the overlying contacting zone to substantially completely coalesce within the layer. Such provision insures that the discontinuous phase liquid will be essentially free of continuous phase liquid before again being discharged as droplets, thereby avoiding undue entrainment of the continuous phase liquid which may otherwise result in substantial backmixing of the latter.
Under the above backmixing conditions, the entrained continuous phase liquid is recirculated together with the discontinuous phase to the lower contacting zone through which the former phase has previously been passed. Backmixing of the discontinuous phase liquid can also occur in the sieve tray system if the linear velocity of the continuous phase through the upcomers is not sufficiently low to prevent entrainment of the discontinuous phase. Backmixing increases the liquid loading of the trays in the column and reduces the overall contacting efficiency of the system. When backmixing occurs to an excessive degree, the discontinuous and continuous phases may become interspersed in the contacted streams withdrawn at the respective ends of the column. The column is then said to be "flooded" and the system flow rates must be substantially reduced before proper flow conditions can be re-established.
In connection with the maintenance of a sufficient height of discontinuous phase liquid on the tray, it is desirable to avoid the occurrence of significant gradients in the discontinuous phase layer on the tray, such as may give rise to short-circuiting liquid flows through the perforated deck. Based on the foregoing, then, it is apparent that the coalescent discontinuous phase liquid layer on the sieve tray is preferably characterized by a height which is sufficient to allow essentially complete settling of the discontinuous liquid and disengagement of the continuous phase liquid therefrom, together with a substantially uniform distribution of the liquid layer on the tray deck to minimize adverse hydraulic effects which tend to lower the tray efficiency.
In the aforedescribed sieve trays, the height of the coalescent discontinuous phase liquid layer on the tray deck is determined by an overall pressure drop which is associated with the separate continuous and discontinuous liquid phases passing through the upcomer means and perforated deck of the tray, respectively. Accordingly, to assure stability of the tray during operation, it is necessary to design the tray deck with sufficient aggregate open area in the perforations thereof to accommodate variations in the liquid loading which may occur from one tray to another in the column while maintaining the constituent pressure drop across the tray deck at a level consistent with the desired height of coalescent liquid on the tray. Similarly, it is also necessary to design the upcomer means so that an adequate continuous phase pressure drop is provided in operation despite variations in liquid loading, while simultaneously maintaining the linear velocity of the continuous phase liquid flowing through the upcomer at a sufficiently low level to prevent entrainment of discontinuous phase liquid droplets from the lower contacting zone in the liquid passing through the upcomer to the upper contacting zone.
Sieve trays presently employed in the liquid-liquid contacting art are frequently designed so that the upper of any two adjacent trays has an imperforate portion of the tray deck positioned above the upcomers of the tray below. These imperforate areas serve to prevent droplets of discontinuous phase liquid from falling into the upcomer of the tray below. Trays of the prior art are typically constructed with upcomers sized so that the continuous phase liquid velocity in the upcomer will be lower than the terminal velocity of some arbitrarily small discontinuous phase liquid droplet, to prevent entrainment of the discontinuous phase from a lower contacting zone in the continuous liquid flowing through the upcomer to an upper adjacent contacting zone. Thus, discontinuous phase droplets falling into the upcomer from an overlying tray deck will bypass the lower tray. The provision of imperforate tray deck areas above upcomers of the adjacent lower tray thus prevents such bypassing and permits the upcomer to function as a settling or disengagement zone wherein discontinuous phase liquid from an underlying contacting zone separates from the continuous phase liquid being transferred, so that only the continuous phase liquid, substantially free of the discontinuous phase, is flowed to the contacting zone overlying the tray.
The foregoing arrangement, while effective in transferring essentially only continuous phase liquid between adjacent contacting zones, is characterized by large imperforate tray areas which constitute a significant portion of the total tray deck cross-sectional area. These imperforate areas thus decrease the amount of tray deck which can be utilized in the formation of droplets from the coalescent layer of discontinuous phase liquid on the tray. As a result, the provision of such imperforate areas to "shield" upcomers of the underlying tray tends to increase the size of the column which is necessary to achieve a requisite level of contacting in a given system.
Another type of widely employed prior art sieve tray involves a sandwich tray assembly comprising two superimposed closely-spaced perforated decks, each deck having a different open area. In such a tray assembly, fluid passes first through the upper deck having the lesser open area and subsequently through the lower deck having the greater open area, so that the first deck is associated with the greater portion of the total pressure drop across the tray assembly. In this manner, the assembly is intended to provide a higher liquid interface above the upper deck, which in turn allows more time for the continuous phase to disengage from the coalescing discontinuous phase and reduces undesirable backmixing. Nonetheless, such design significantly increases the cost as well as the complexity of the tray relative to those employing a single perforated deck.
From a structural standpoint, it has been the practice of the prior art to dispose beams extending transversely across the tray for support and mechanical strength. This is practical particularly in large scale columns, where many upcomers or downcomers are employed to reduce the lateral hydraulic and concentration gradients across the tray. However, such support members also occlude large portions of the tray surface, so that a significant amount of the tray's cross-sectional area is lost for contacting purposes.
Accordingly, it is an object of the present invention to provide an improved liquid-liquid contacting sieve tray which is easily and inexpensively fabricated, which utilizes a larger part of the tray deck for discontinuous phase liquid collection, coalescence and droplet formation and which is characterized by stable operation, uniform coalescent liquid distribution on the tray deck and high resistance to backmixing behavior under conditions of variant liquid flows.
It is a further object of the invention to provide an improved tray of the above type which is characterized by a small cross-sectional area requirement.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.